ACL2 supports input and output facilities equivalent to a subset
of those found in Common Lisp. ACL2 does not support random access
files or bidirectional streams. In Common Lisp, input and output
are to or from objects of type stream. In ACL2, input and output
are to or from objects called ``channels,'' which are actually
symbols. Although a channel is a symbol, one may think of it
intuitively as corresponding to a Common Lisp stream. Channels are
in one of two ACL2 packages, "ACL2-INPUT-CHANNEL" and
"ACL2-OUTPUT-CHANNEL". When one ``opens'' a file one gets back
a channel whose symbol-name is the file name passed to ``open,''
postfixed with -n, where n is a counter that is incremented
every time an open or close occurs.

There are three channels which are open from the beginning and which
cannot be closed:

All three of these are really Common Lisp's *standard-input* or
*standard-output*, appropriately.

For convenience, three global variables are bound to these rather
tedious channel names:

*standard-ci*
*standard-oi*
*standard-co*

Common Lisp permits one to open a stream for several different
kinds of io, e.g. character or byte. ACL2 permits an additional
type called ``object''. In ACL2 an ``io-type'' is a keyword, either
:character, :byte, or :object. When one opens a file, one specifies
a type, which determines the kind of io operations that can be done
on the channel returned. The types :character and :byte are
familiar. Type :object is an abstraction not found in Common Lisp.
An :object file is a file of Lisp objects. One uses read-object to
read from :object files and print-object$ to print to :object files.
(The reading and printing are really done with the Common Lisp read
and print functions.)

File-names are strings. ACL2 does not support the Common Lisp type
pathname.

Here are the names, formals and output descriptions of the ACL2 io
functions.

The ``formatting'' functions are particularly useful;
see fmt and see cw. In particular, cw prints to a
``comment window'' and does not involve the ACL2 state, so many may
find it easier to use than fmt and its variants. The functions
fms!, fmt!, and fmt1! are the same as their respective functions
without the ``!,'' except that the ``!'' functions are guaranteed to
print forms that can be read back in (at a slight readability cost).

When one enters ACL2 with (lp), input and output are taken from
*standard-oi* to *standard-co*. Because these are synonyms for
*standard-input* and *standard-output*, one can drive ACL2 io off of
arbitrary Common Lisp streams, bound to *standard-input* and
*standard-output* before entry to ACL2.

By default, symbols are printed in upper case when vertical bars are
not required, as specified by Common Lisp. As with Common Lisp,
ACL2 supports printing in a "downcase" mode, where symbols are
printed in lower case. Many printing functions (some details below)
print characters in lower case for a symbol when the ACL2 state
global variable print-case has value :downcase and vertical bars
are not necessary for printing that symbol. (Thus, this state global
functions in complete analogy to the Common Lisp global
*print-case*.) The value print-case is returned by
(acl2-print-case), and may be set using the macro
set-acl2-print-case (which returns state), as follows.

The ACL2 user can expect that the :downcase setting will have an
effect for formatted output (see fmt and see fms) when the
directives are ~p, ~P, ~q, or ~Q, for built-in functions princ$ and
prin1$, and the ppr family of functions, and not for built-in
function print-object$. For other printing functions, the effect of
:downcase is unspecified.